R&AW don't engage in direct assassinations. But they do employ a tactic of killing off people with the help of rival gangs. In Canada's case, it is entirely possible we paid off some rival gang in Punjab to kill this Nijjar guy. Similar to the case of Pakistan, you can find gunman for hire in Pakistan due to their economic situation, and R&AW is using that opportunity to the maximum. As far as we know, our foreign intel agency is not like ISI a military organization, but rather a civilian one with police officers as well as direct recruits leading the charge.